Such a a pile foundation structure conventionally known is a structure illustrated in FIG. 4.
In other words, it has a structure wherein a point pile or a friction pile 82 as a structural element, is driven into the underground 81, such as solid rock, a bottom of a footing 83 is mounted on the head of the pile 82, and the pile 82 and the footing 83 are rigidly bound by a reinforcing steel members 84 including a pile reinforcement and a concrete-reinforcing bar and concrete.
In the case of the conventional pile foundation structure as mentioned above, having an arrangement wherein the pile 82 and the footing 83 are rigidly bound, when an external force such as an earthquake is applied, the underground 81 including the ground and the upper structure are separately moved, whereby a load including a shearing force and a bending moment or the like is concentrated on a binding portion of the head of the pile being a boundary portion therebetween. Consequently, a problem arises wherein the head of the pile 82 and the footing 83 maybe damaged and even destroyed. The pile foundation structure itself is a bottom portion structure supported by the pile 82 of the structure element driven into the underground. As a result, it is extremely difficult and requires a long period and huge costs to conduct a restoration construction in case damage or a destruction is caused on the head of the pile 82 and the footing 83.
In order to resolve the above problems, the pile foundation structure employing a sliding structure has been suggested in Japanese Patent Laid-open No. 1-102124.
In other words, the pile foundation structure is like that shown in FIG. 5; and structurally described below. A number of steel reinforcement members 92 annularly disposed on an upper end of a concrete pile 91, and steel pipes 93 surrounding the steel reinforcement members 92 in a state of allowing lateral movement of the steel reinforcement members 92 are upwardly extended. At the lower end portion of the steel pipes 93, annular engagement projections 94 are arranged so as to be connected to an upper end portion of the concrete pile 91 in a buried state. Moreover, a top board 95 is welded to the upper end portion of the steel pipes 93. The top board 95 is provided with flexible holes 96 for making the steel reinforcement members 92 pierce the top board 95 so as to respectively allow lateral shaking of the steel reinforcement members 92. The steel reinforcement members 92 protruding above the top board 95 through the flexible holes 96, are connected to the concrete structure body (or the footing) 97 of the upper portion thereof. Furthermore, there is interposingly a sliding member 99 between the top board 95 and a footing metal member 98, through which the concrete structure body 97 is slidably mounted on the top board 95.
In the pile foundation structure having such a sliding arrangement, when an external force, such as an earthquake, is applied, the steel reinforcement members 92 are bent, and at the moment while they are bent, the sliding member 99 makes the concrete structure body 97 slidingly move with respect to the concrete pile 91, thereby making it possible to restrict the load concentration of the bending moment or the like on the binding portion of the pile head. However, a sliding amount is regulated by the size of each flexible hole 96 disposed on the top board 95, and the concrete pile 91 is connected to the concrete structure body 97 by the steel reinforcement members 92, whereby a shearing force and the bending moment cannot be efficiently absorbed, in the case of applying the excess external force such as the earthquake. Consequently, it is unavoidable that the head of the pile 91 and the concrete structure body 97 is damaged or destroyed. In the same way as the conventional pile foundation structure in FIG. 4, there has been a problem wherein the earthquake-proof property and earthquake avoidable property cannot be ensured enough.